As a method for generating a stable voltage lower than an input voltage, non-insulating type step-down chopper circuits are widely used.
For example, JP-A-2010-74972 discloses a non-insulating type step-down chopper circuit, which supplies a high gate drive voltage to a gate terminal of a metal-oxide semiconductor field-effect transistor (MOSFET) arranged on the high side until the MOSFET disposed on the high side turns on and decreases the gate drive voltage when the MOSFET is in the ON state, thereby suppressing noises generated in an output voltage.
In a non-insulating type step-down chopper circuit, at the moment when a gate drive voltage of a MOSFET disposed on the high side rises and arrives at a threshold voltage of the MOSFET so as to cause the MOSFET to be the ON state, most of a gate charging current flows through a drain terminal through drain-to-gate feedback capacitance of the MOSFET. Therefore, a level difference occurs in the waveform of a gate voltage of the MOSFET disposed on the high side, and the on-resistance of the MOSFET disposed on the high side rapidly increases. As a result, a counter-electromotive force is generated in the input terminal of the step-down chopper circuit in accordance with a parasitic inductor generated in an input wiring.
As the gate drive voltage becomes high with respect to the threshold voltage of the MOSFET disposed on the high side, a change in the on-resistance of the MOSFET increases. Therefore, the generated counter-electromotive force tends to increase. According to the counter-electromotive force, there is a possibility that a noise is generated in the output voltage or the circuit is damaged.
In order to suppress the generation of the counter-electromotive force, it is effective to configure the inclination of a rising edge of the waveform of the gate voltage of the MOSFET disposed on the high side to be gentle. However, according to such a method, a switching loss increases, and thus the power conversion efficiency decreases.
In the step-down chopper circuit described in JP-A-2010-74972, until the MOSFET disposed on the high side is in the ON state, a high gate drive voltage is supplied to the gate terminal. For this reason, an increase in a difference between the threshold voltage and the gate drive voltage of the MOSFET disposed on the high side cannot be avoided, and the generation of a counter-electromotive force cannot be suppressed.